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Eggs incubated in the presence of the artificial light at night (ALAN) had a zero success rate of hatching, resulting, of course, in no surviving offspring. This compares to an 86% success rate for eggs incubated in the replicated day-night light settings.

In other words, ALAN appears to disguise natural light cues that trigger clownfish eggs to hatch after dark.

“When ALAN is present, no eggs hatched but when the light was removed during the recovery period, eggs from the ALAN exposure hatched like normal, so the presence of light is clearly interfering with an environmental cue that initiates hatching in clownfish,” says lead researcher Emily Fobert.

The researchers say that other fish species that hatch during the day, such as the Japanese rice fish (Oryzias latipes) and Atlantic salmon (Salmo salar), may hatch outside of optimal hatching times, compromising the survival of the young.

“These findings likely extend to other reef fish as many share similar reproductive behaviours, including the timing of hatching during early evening,” says Fobert.

Her colleague Karen Burke da Silva, who is also a founder of the Saving Nemo Conservation Fund, says the impact of ALAN on coral reefs needs to be understood to help already stressed ecosystems.

“Artificial light as night is becoming a greater concern among ecologists, as light is spreading globally, and the impacts on organisms can be severe, but very little research has been done around ALAN in the marine environment,” she says.